1. Field of the Invention
The present invention relates to a method of controlling a D.C. motor by using piezoelectric resistance effect exhibited by a semiconductor and also to an apparatus which performs such a control.
2. Description of the Related Art
FIG. 5 illustrates a conventional motor control apparatus. The apparatus has a disk 3 connected to the drive shaft 2 of a DC motor 1. The disk 3 is provided with a multiplicity of slits 3a arranged along the outer peripheral end of the disk 3 at a constant circumferential pitch. A photo-interrupter 4 is disposed in the vicinity of the disk 3 so as to oppose the slits 3a. An integrator 7 is connected to the photo-interrupter 4 through an amplifier 5 and a monostable circuit 6. The integrator 7, as well as a power supply 9, is connected to a differential amplifier circuit 8 for controlling the voltage applied to the motor 1.
In operation, the motor 1 is supplied with a certain level of voltage applied by the power supply 9 through the differential amplifier circuit 8. As the motor 1 is started, the disk 3 is rotated by the drive shaft 2 connected to the motor 1, so that a pulse train P.sub.A indicative of the presence or absence of the slits 3a is output from the photo-interrupter 4. The pulse train P.sub.A, which is amplified by the amplifier 5, is input to the monostable circuit 6 as a trigger pulse. In consequence, a pulse train P.sub.B, having a frequency proportional to the speed of the motor 1 and having a constant high-level period, is delivered to the integrator 7.
If the time constant of the integrator 7 is sufficiently greater than the period T of the pulse train P.sub.B, a DC voltage V.sub.0 expressed by the following formula is delivered from the integrator 7. EQU V.sub.0 =.alpha..multidot.t.sub.p /T=.alpha..multidot.t.sub.p .multidot.f(1 )
where, .alpha. represents a constant, t.sub.p represents the high-level period of the pulse train P.sub.B and f represents the frequency of the pulse train P.sub.B.
In formula (1) above, since .alpha. and t.sub.p are constant, the D.C. voltage V.sub.0 output from the integrator 7 varies in proportion to the frequency f of the pulse train P.sub.B, i.e., the speed of rotation of the motor 1. Then, the difference between the voltage supplied by the power supply 9 and the output level of the integrator 7 is amplified by the differential amplifier 8 and supplied to the motor 1.
Therefore, an increase in the speed of the motor 1 causes the output of the integrator 7 to increase correspondingly, with the result that the output of the differential amplifier 8 is lowered to reduce the speed of the motor 1. Conversely, a reduction in the speed of the motor 1 causes the output from the integrator 7 to decrease correspondingly, so that the level of the output from the differential amplifier 8 becomes higher to increase the speed of the motor 1.
Thus, the speed of the DC motor 1 is controlled in accordance with the voltage from the power supply 9.
In this known motor control apparatus, however, a problem is encountered in that the motor speed cannot be set with good response when the motor is started up or when the command speed of the motor is changed, because the response characteristic of the motor control is determined by the time constant of the integrator 7.
Another problem is that, when the interval between successive pulses of the pulse train P.sub.B is large due to low speed of operation of the motor 1 or a large distance between adjacent slits 3a in the disk 3, a ripple is caused in the output of the integrator 7 so as to impede smooth control of the motor. This problem would be overcome by setting the time constant of the integrator 7 to a large value. In such a case, however, the response characteristic of the control system responding to a variation in the motor speed is impaired undesirably.